The invention relates to a method for induction bend forming a pressure-resistant pipe having a large wall thickness and a large diameter, in particular a pipe in a power plant or a liquid or gas pipeline.
For carrying liquid and gaseous media under pressure, steel pipes are required that have a large wall thickness in order to withstand the stresses. Such requirements apply, for example, to the transport of hot steam in power plants, where pipe bends are required in order to adapt the pipelines to the constructional circumstances or for transporting crude oil in pipelines over long distances, where flexible U-shaped expansion loops are used at regular intervals to compensate for thermally induced changes in length. To enable a large throughput, a large opening cross-section and correspondingly a large outer pipe diameter is required. The present method relates to pipes with typical nominal diameters greater than 300 mm and a diameter to wall thickness ratio of 10:1 to 100:1, typically 20:1 to 70:1.
Such a method for induction bend forming has long been known, for example from DE 2513561 A1 and has been continually improved in order to produce dimensionally very stable pipe bends despite the enormous dimensions. Forming of such massive pipes can only be achieved by inductively heating a narrow annular zone to a forming temperature above 850° C. Structural changes occur in the material, which is usually fine-grained steel, in the heat-affected zone. In order to homogenize the structure after hot forming and thus improve the mechanical properties of the steel, the pipe bend is subsequently often heat-treated at a temperature of about 600° C. The straight pipe sections, which are connected before and after the pipe bend and are also referred to as tangents, are also influenced by the subsequent heat treatment. However, since they were not heated to a high temperature in the course of the forming process and their structure has, therefore, remained unchanged, the subsequent heat treatment has a negative effect on these sections; they embrittle. Thus, these sections must be separated, and the pipe bend produced by induction bend forming has to be welded to new tangents.
This has disadvantages because of the high work effort, in particular when a plurality of pipe bends, even in different directions, are carried out successively on the same pipe piece, as made possible by the device described in DE 10 2010 020 360 A1. The simplification and acceleration of pipeline construction thus achieved by producing a three-dimensional pipe structure in only one operation is negated if the straight tangent pieces have to be replaced because a thermal post-treatment of the pipe formation necessary in order to achieve certain strength values. To avoid this, only the use of pipes of high-strength steels and/or of greater wall thickness is possible in order to retain the mechanically required minimum strength values for the overall structure after the heat treatment at the tangents. However, this approach is also disadvantageous because of considerably higher material prices.